The present invention relates generally to electrical power distribution equipment. More particularly, the invention relates to sub-assemblies or modules that contain discrete electrical components and that are employed in protective devices such as surge arresters. Still more particularly, the invention relates to apparatus and methods for applying an axially-compressive force to an array of electrical components and retaining those components under compression in end-to-end relationship within the module.
Under normal operating conditions, electrical transmission and distribution equipment is subject to voltages within a fairly narrow range. Due to lightning strikes, switching surges or other system disturbances, portions of the electrical network may experience momentary or transient voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. Left unprotected, critical and costly equipment such as transformers, switching apparatus, computer equipment, and electrical machinery may be damaged or destroyed by such over-voltages and the resultant current surges. Accordingly, it is routine practice within the electrical industry to protect such apparatus from dangerous over-voltages through the use of surge arresters.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert the over-voltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When caused to operate, a surge arrester forms a current path to ground having a very low impedance relative to the impedance of the equipment that it is protecting. In this way, current surges which would otherwise be conducted through the equipment are instead diverted through the arrester to ground. Once the transient condition has passed, the arrester operates to open the recently-formed current path to ground, thereby again isolating the distribution or transmission circuit in order to prevent the non-transient current of the system frequency from "following" the surge current to ground, such system frequency current being known as "power follow current."
Conventional surge arresters typically include an elongate outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and ground, and an array of other electrical components forming a series path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements. These nonlinear resistors or "varistors" are characterized by having a relatively high resistance at the normal steady-state voltage and a much lower resistance when the arrester is subjected to transient over-voltages. Depending on the type of arrester, it may also include one or more spark gap assemblies housed within the insulative enclosure and electrically connected in series with the varistors. Some present-day arresters also include electrically conducting spacer elements coaxially aligned with the varistors and gap assemblies. Electrodes of a variety of types and configurations may also be included in the component array in conventional arresters.
For an arrester to function properly, it is important that contact be maintained between the ends of the various surge arrester components in the array. To accomplish this, an axial load is placed on the elements in the array. Such loading is typically applied by employing springs within the housing to urge the stacked elements into engagement with one another. Good axial contact is important to ensure a relatively low contact resistance between the adjacent faces of the components, to ensure a relatively uniform current distribution through the elements, and to provide good heat transfer between the arrester elements in the array and the end terminals.
Another conventional means for supplying the required axial force is to wrap the stack of arrester elements with glass fibers so as to axially-compress the elements within the stack. Examples of such prior art surge arresters include U.S. Pat. Nos. 5,043,838, 5,138,517, 4,656,555 and 5,003,689. These patents generally describe rather elaborate techniques for winding the fibers about the ends of a stack of arrester components to apply the appropriate axial force to the components within the stack. Employing certain of these techniques requires the inclusion of specially-configured components within the stack, such as special end terminations for maintaining specific separations between the fibers (for example, U.S. Pat. No. 5,043,838) or for creating a shoulder against which the fibers can be wound (for example, U.S. Pat. No. 5,138,517).
In addition to maintaining an axial compression, these stacked arrester components must be retained in such a manner that will permit gases evolved during arrester failure to be safely vented from the arrester. Occasionally, a transient overvoltage condition may cause some degree of damage to one or more of the resistive elements. Damage of sufficient severity can result in arcing within the arrester housing, leading to extreme heat generation and gas evolution as the internal components in contact with the arc are vaporized. This gas evolution causes the pressure within the arrester to increase rapidly until it is relieved by either a pressure relief means or by the rupture of the arrester housing. The failure mode of arresters under such conditions may include the expulsion of components or component fragments at high velocities and in all directions. Such failures pose potential risks to personnel and equipment in the vicinity.
Attempts have been made to design and construct arresters that will not catastrophically fail with the expulsion of components or component fragments. One such arrester is described in U.S. Pat. No. 4,404,614 which discloses an arrester having a non-fragmenting liner and outer housing, and a pressure relief diaphragm located at its lower end. A shatterproof arrester is also disclosed in U.S. Pat. Nos. 4,656,555, 4,930,039 and 5,113,306. Arresters having pressure relief means formed in their ends are described in U.S. Pat. Nos. 3,727,108, 4,001,651, and 4,240,124. U.S. Pat. No. 5,043,838 discloses a filament wrapped arrester module that includes openings between the crisscross pattern of windings. These openings are filled with an epoxy or similar insulating material that is permitted to rupture to allow the expulsion of gasses.
Despite such advances, however, state of the art arresters may still occasionally fail with the expulsion of components or fragments of components. This may, in part, be due to the fact that once the internal components in these arresters fail, the resulting arc vaporizes the components and generates gas at a rate that cannot be vented quickly enough to prevent rupture of the arrester enclosure. Accordingly, there remains a need in the art for an arrester which, upon failure, will fail in a non-fragmenting and safe manner. A need also exists for an arrester whose components are axially compressed without the use of a spring.
There further remains a need in the art for a means to compress axially an array of arrester components that may be applied simply and easily, without elaborate and costly manufacturing procedures or the addition into the component stack of specialized components. Preferably, the means would be easily applied to the external surfaces of the stacked components. It would be further advantageous if the compression means were to include features enhancing the tensile and cantilever strengths of the arrester assembly. Further, the device should provide a venting means for relieving gas pressure and preventing the electrical assembly from failing in a dangerous fashion, and should provide good bonding at each interface from the MOV stack outward without requiring complicated assembly procedures or costly waste.